The exhaust gas system Is mechanically de-coupled by means of the de-coupling element for providing a certain degree of flexibility over the length of the vehicle. As mechanical de-coupling elements non-self-supporting, flexible connecting elements, such as corrugated tubes or flexible, tubes are, inserted between two sections of the exhaust gas system. Due to the fact that the corrugated tubes or flexible tubes are hot self-supporting, they must be supported. Owing to their small degree of stiffness and flexibility, they have also the inherent property of absorbing to a certain extent vibrations in certain frequency ranges. Such flexible sound-absorbing insulating elements are described for instance in U.S. Pat. No. 5,456,291 A and in EP 1 431 538 B1.
For a mechanical articulated de-coupling of two pipe flanges or casing flanges of an exhaust gas system which permits a relative bending of the exhaust gas system, also jolted connecting elements which have a joint-like flexibility are inserted between two sections. These de-coupling elements which are described in DE 198 12 611 C2 or in JP 199789173 A (Hei9-89173) are shaped as a jolted pipe length which forms in the longitudinal section an S-shaped spring. Owing to the reduced wave shape the said connecting elements are stiff when compared to the corrugated tubes; they are, however, self-supporting. In order to increase their flexibility they can be provided with a longitudinal slot according to DE 198 12 611 C2.
It is known that due to most different modern construction methods for exhaust gas systems and internal combustion engines, an increasing number of vibrations in the audible range are caused, in spite of a mechanical de-coupling, by resonance in different components. Flatter forms of mufflers, the use of thin-wall, ventilation-slot-insulated sheet metal manifolds and turbochargers in diesel engines as well as changes in the combustion process result in additional vibrations in the structures and thus in additional solid-borne vibrations.
This kind of solid-borne vibration may be reduced directly in the structures by means of sound-absorbing insulating elements in the exhaust gas system or also by great impedance leaps in components of an exhaust gas system.
According to DE 10 2006 040 980 A1 impedance leaps are achieved with a sound-absorbing insulating element provided with radial widenings of the cross-section which are attached onto the pipe like a pipe clamp.
In DE 20 2004 005 526 U1 a rigid flanged joint is described for the reduction of solid-born vibrations which muffles the transmission of vibrations by means of a line-shaped structure in the circumferential direction around the pipe axis or flange axis.